Light has been the probe of choice to investigate and modify properties of matter. The development of ever more powerful light sources is the key to sustained progress in that field. Besides lasers, synchrotron radiation has played a growing role since the 1970s. The undulator is the predominant source type employed in the modern 3rd and 4th generation synchrotron light sources and Free Electron Lasers (FEL). Undulators are magnetic devices that generate a spatially periodic magnetic field variation that causes a charged particle beam, usually electrons, to emit electromagnetic radiation with special properties. Undulators are the prime magnetic devices for the generation of highly brilliant synchrotron light by the 3rd and the 4th generation light sources. The development of undulators with higher magnetic field and smaller magnetic period in the mm range is an important technical problem under study currently. The motivation to build such miniundulators is to produce harder radiation for a given beam energy or to save accelerator cost by using a lower electron energy for a given photon energy.
In principle, short period undulators can be built in various ways: they can be Halbach-type undulators with permanent magnets, hybrid-type undulators, or the so-called electromagnetic undulators. In Halbach-type undulators and hybrid undulators, the maximum field is mainly defined by the material properties of the rare earth magnets and, to a certain extent, by the specific design details. They are difficult to build with high peak field when the period length is in the mm-region. Electromagnetic undulators have the disadvantage that both the required currents as well as the Ohmic losses are relatively high. The use of superconductors instead of normal conductors reduces the Ohmic losses to a negligible amount. For this reason, around 1990, both Brookhaven (Ben-Zvi, Z. Y. Jiang, G. Ingold and L. H. Yu, Nucl. Instrum. Methods, A 297, 301 (1990)) and Karlsruhe (H. O. Moser, B. Krevet and H. Holzapfel, Forschungszentrum Karlsruhe, German Patent P 41 01 094.9-33, Jan. 16, 1991) presented different proposals to replace the permanent magnets by superconducting wires or striplines in order to increase the field strength of the undulators. They combined the advantages of superconductivity and in vacuo design, and it was demonstrated that the field strength with superconducting undulators can be significantly higher in comparison with conventional undulators.
Superconducting miniundulators have the potential to overcome some limitations of conventional undulators. They are expected to play an important role in upgrade projects of 3rd generation sources and FELs. In the past, there has been considerable progress in developing superconducting miniundulators at several places.
Up-to-date three different superconducting coil arrangements have been used for a planar superconducting undulator. The general design goal is to reduce undulator period length as much as possible while maintaining its undulator parameter K close to 2 in the interest of tunability. K is given asK=0.934B0[T]λu[cm]  (1)with B0 the peak field on axis in Tesla and λu the undulator period length in cm. As long as the K parameter can go up to 2, the undulator is fully tunable which means that the whole range from its fundamental frequency to say 7th or even higher harmonics can be scanned.
Referring to FIG. 1A, in one superconducting coil arrangement, a superconducting solenoid is cut in two identical shorter solenoids 100, and the shorter solenoids are pulled apart to get a so-called split-pair solenoid. A number of split-pair solenoids are then placed next to each other with alternating field direction in a row to obtain the spatially alternating field of an undulator. In an undulator according to FIG. 1A, the particle beam travels perpendicularly to the solenoid axis through the gap created by splitting the solenoids. However, this concept cannot be easily miniaturized. Referring to FIG. 1B, another approach is to realize a meandering current path in one plane (slab) 104 and arranging two such slabs opposite one another and separated by a small gap, in which the spatially alternating field is generated. This design can be implemented in micro manufacturing and, thus, is a way to realize micrometer scale period length.
Referring to FIG. 1C, in yet another solution, two solenoids 102 are wound bifilarly (pair of conductors with opposite current direction) and placed parallel to one another with a small gap therebetween. In this arrangement, the magnetic field vanishes more or less within solenoids 102 and also far away because of the opposite current direction. But, in the immediate neighborhood at the outside surface of solenoids 102, in particular, between the solenoids in the gap g, the field is strong. The cross section of solenoids 102 in this case is no longer circular; it has at least one almost straight section which forms the gap for the particle beam. Rather short periods are possible with this set up. In this design, the particle beam travels in the gap parallel to the axis of the two solenoids 102 and the peak field is high compared to permanent magnet set-ups and scales with the gap g.
FIG. 2 depicts a 3-D schematic of the superconducting miniundulator based on the design shown by FIG. 10. The design consists of two ferromagnetic cores 106 with superconducting coils 108 that are placed symmetrically with respect to the midplane of the superconducting undulator in which a particle beam 109 travels. The flat sides of cores 106 adjacent to the midplane are the top and bottom undulator poles. Each core 106 includes grooves for receiving a respective superconducting coil 108. The design of FIG. 2 allows for the application of two types of beam vacuum systems. In the case of low power single-pass beams, the beam can share the vacuum with the superconducting coils. In the case of storage rings in which the electron beam chamber vacuum needs to be of the order of 10−9 mbar, a shared insulation and beam vacuum is excluded due to materials like the insulation of superconducting wires and the many cryogenic structural materials. So the beam should use a separate vacuum chamber, typically with an elliptical cross section, that will be inserted in the gap between the poles. The superconducting coils 108 need to be maintained at temperatures around 4 K when conventional superconducting wires made of NbTi are used. For other superconducting wire materials, in particular, for high Tc superconductors, higher temperatures may be exploited, e.g., around the temperature of liquid nitrogen 77 K. The most straightforward solution for the cooling of medium size superconducting undulators is the usage of liquid helium. Another solution is cryogen-free cooling, which is based on commercially available two-stage cryocoolers and massive copper leads to connect the cold head of the cryocooler with the coil to be cooled.
A well known superconducting material for coils 108 is NbTi. Moreover, some laboratories have done prototyping work with Nb3Sn in order to benefit from the higher critical current. Compared to NbTi, a magnetic field increase by about 30-50% is expected from Nb3Sn conductors. Other superconductors are being observed for their suitability, in particular, high Tc superconductors. Normally, a rectangular wire will be used for coils 108 instead of a round wire, because a larger packing factor and better control of the wire positioning in the grooves can be achieved.